This study will examine the safety and effectiveness of a single dose of anti-interleukin-5 antibody (SCH 55700) in reducing the number of eosinophils (a type of white blood cell) in patients with hypereosinophilic syndrome. Patients with this disease have too many eosinophils in the blood and in body tissues, which cause damage to the affected organs-most commonly the heart, nerves and skin. SCH 55700 is an antibody to interleukin 5-a hormone-like substance produced by white blood cells that plays a significant role in eosinophilia. SCH 55700 has lowered eosinophilia blood counts in patients with asthma and reduced the number of these cells in tissues of animals with asthma.